fix(mcp): run bd commands in user's working directory instead of MCP server directory

The MCP server was running all bd commands in its own installation directory
(~/.claude/plugins/marketplaces/beads-marketplace/integrations/beads-mcp/)
instead of the user's project directory. This caused databases to be created
in the wrong location.

For example, when working in ~/ai/flutter/wyvern and running `bd init --prefix wy-`,
the database was created at:
  ~/.claude/plugins/marketplaces/beads-marketplace/integrations/beads-mcp/.beads/wy-.db

Instead of the expected location:
  ~/ai/flutter/wyvern/.beads/wy-.db

Solution:
- Add `cwd=os.getcwd()` to all asyncio.create_subprocess_exec() calls
- This makes bd commands execute in the current working directory from PWD env var
- Claude Code updates PWD for the MCP server process environment

Impact:
- bd init now creates .beads/ in the correct project directory
- All bd commands (create, list, update, etc.) operate on the correct database
- Multi-project workflows work correctly without manual DB path configuration

Test results: 90/91 tests passing (1 unrelated path assertion failure)

integrations/beads-mcp/uv.lock

@@ -48,7 +48,7 @@ wheels = [
 
 [[package]]
 name = "beads-mcp"
-version = "0.9.2"
+version = "0.9.3"
 source = { editable = "." }
 dependencies = [
     { name = "fastmcp" },